1. Field of the Invention
The present invention relates to a cross coil type indicator in which a magnet rotor is rotated by a resultant magnetic field generated by cross coils arranged in quadrature with each other and a detected variable is indicated by a pointer rotated together with the rotor.
2. Description of the Prior Art
As shown in FIG. 1, the pointer driving portion of the cross coil type indicator is, in general, comprised of a pair of coils L.sub.1 and L.sub.2 arranged in quadrature with each other, a magnet rotor Mg rotatively provided in a resultant magnetic field generated by the two coils L.sub.1 and L.sub.2, a pointer A provided at one end of the rotary shaft (not shown) mounted on the magnet rotor Mg, a scale plate B for indicating the detected variable in cooperation with the rotating position of the pointer A, a zero point stopper pin P which restricts the rotation of the pointer A, and a spiral spring C for biasing the pointer A and the magnet rotor Mg toward the stopper pin P.
With this construction, when drive currents i.sub.1 and i.sub.2 are flowed through the cross coils L.sub.1 and L.sub.2 in quadrature as shown in FIG. 2, respectively, magnetic fields generated in each of the coils can be represented by the following equations; ##EQU1## Here, .mu..sub.1 and .mu..sub.2 indicate each magnetic permeability of the coils L.sub.1 and L.sub.2, respectively,
n.sub.1 and n.sub.2 indicate the number of windings of the coils, respectively, and PA1 S.sub.1 and S.sub.2 indicate each cross-sectional area of the coils, respectively.
On the other hand, since the magnet rotor Mg is rotated in the resultant magnetic direction of the magnetic fields .phi..sub.1 and .phi..sub.2, its rotation angle .alpha. can be represented by the following equation; ##EQU2## When the coils L.sub.1 and L.sub.2 having an equal structure respectively is used, the equation (2) can be rewritten as follows; ##EQU3##
That is to say, the rotation angle .alpha. is defined by the drive currents i.sub.1 and i.sub.2.
Supposing that the drive currents i.sub.1 and i.sub.2 are ##EQU4## then, the rotation angle .theta. corresponding to the resultant magnetic field can be represented as follows; ##EQU5##
Namely, the rotation angle .theta. becomes equal to the rotation angle of the pointer, i.e., .alpha.=.theta.. As a result, the direction of the resultant magnetic field generated by the cross coils L.sub.1 and L.sub.2 becomes equal to the angle .theta. as shown in FIG. 3 and the magnet rotor Mg is rotated up to the rotation angle .theta. against the biasing force of the spiral spring C, which corresponds to the detected variable.
Accordingly, by reading the position of the pointer A which rotates together with the magnet rotor Mg with respect to the scale plate B, it becomes possible to know the value of the detected variable. It is to be noted that the amplitudes of the drive currents i.sub.1 and i.sub.2 in this case are determined by taking into consideration of the biasing force of the spiral spring C.
In order to drive the cross coils as set forth above, it is necessary to provide a drive unit for driving the cross coils. In this case, it is preferable for the drive unit to be able to apply pulse currents of sin .theta. and cos .theta. having duty ratios corresponding to the detected variable to the cross coils as the drive currents. Further, in order to lower production cost of the indicator, it is requested for the drive unit to have a simple structure. Furthermore, if there is provided such a drive unit in an cross coil type indicator, several problems coused in a conventional cross coil type indicator can be settled since it becomes possible to make degital processing of various data.
Hereinafter, one of the problems will be described with reference to the drawings.
FIGS. 4 to 6 show one of prior art indicators having a drive unit which can apply pules currents having duty ratios corresponding to the detected variable.
Namely, in the drive unit of the cross coil type indicator as shown in FIG. 4, pulse currents having duty ratios corresponding to the detected variable are generated, and they are applied to the cross coils L.sub.1 and L.sub.2 through an output circuit 50.
The drive unit comprises a sensor 10 for detecting a variable such as car speed, a frequency/voltage (F/V) converter 20, a duty ratio calculating circuit 30, a duty pulse generating circuit 40 and an output circuit 50. The cross coils L.sub.1 and L.sub.2 and the magnet rotor Mg are the same as those indicated in FIG. 1.
In operation, an output signal such as vehicle speed from the sensor 10 is applied to the frequency/voltage (F/V) converter 20, where the frequency of the output signal is converted into a D.C. voltage. The D.C. voltage thus converted is applied to a duty ratio calculating circuit 30, where sin .theta. and cos .theta. of the rotation angle .theta. of the pointer A are sought in accordance with the voltage. Then, each duty ratio shown in FIG. 5 is calculated therein from duty characteristics which change in accordance with sin .theta. and cos .theta.. Each of the duty pulses is generated in the duty pulse generating circuit 40 and pulse currents having duty ratios are applied to the coils L.sub.1 and L.sub.2 through the output circuit 50, respectively.
Since the pulse currents i.sub.1 and i.sub.2 are proportional to sin .theta. and cos .theta. and the angle .alpha. of the resultant magnetic field is changed in accordance with the detected variable, the pointer A is rotated against the biasing force of the spiral spring in the direction of the resultant magnetic field vector, thus enabling the read-out of the rotation angle of the pointer A from the scale plate B.
Now, as stated previously a stopper pin P is provided at a position near the zero indicating position as shown in FIG. 1 and the pointer A is normally biased by the spiral spring C toward the stopper pin P. As a result, when the pointer A is situated near the zero indicating position in the case of a speed meter or "E" position in the case of a fuel indicator, the pointer A comes into contact with the stopper pin P. Since a desirable linearity in indication cannot be maintained in the vicinity of "zero" or "E" position on the scale plate B, the "zero" or "E" position has to be shifted to a higher position which is normally about 5.degree. to 10.degree. above from its actual position, and the stopper pin position must also be shifted to the corresponding position. Because of this situation, noise is generated by repeated abutments between the pointer and stopper pin due to the reasons which will be explained later.
FIG. 6 indicates a detailed circuit construction of the output circuit 50 shown in FIG. 4. In the output circuit 50, two pairs of the NPN transistors Q11, Q13 and Q12, Q14 are connected in the form of a first bridge circuit together having the first coil L.sub.1, and another two pairs of the NPN transistors Q21, Q23 and Q22, Q24 are connected in the form of a second bridge circuit having the second coil L.sub.2 between the power supply +Vcc and the ground. Each base of the transistors Q11 through Q24 is connected to the duty pulse generating circuit 40.
In operation, when each of the pulse currents i.sub.1 and i.sub.2 is desired to flow through the coils L.sub.1 and L.sub.2 in the direction of the solid line shown in FIG. 6, the transistors Q11, Q14 are turned on and the transistors Q21, Q24 are turned off, while the transistors Q21, Q24 are turned on and the transistors Q22, Q23 are turned off by the application of pulse signals to the base of each transistor from the duty pulse generating circuit 40.
On the other hand, when the pulse currents i.sub.1 and i.sub.2 as the drive currents are desired to flow through the coils L.sub.1 and L.sub.2 in the direction of the dotted line shown in FIG. 6, the transistors Q12, Q13 are turned on and the transistors Q11, Q14 are turned off while the transistors Q22, Q23 are turned on and the transistors Q21, Q24 are turned off.
Since the pointer A is driven by the pulse currents flowing through the coils L.sub.1 and L.sub.2, which are responsive to the changes in the duty ratios or the detected variable, the pointer A is subject to a vibration generated by the changes and therefore noise is generated when the pointer A strikes the stopper pin P. Especially, when the indicator is applied to a speed meter, under the low speed condition the pointer A is constructed in such a manner that it never comes off from the stopper pin so that on and off action is repeatedly happened between the pointer A and the stopper pin P at a low speed condition due to the vibration in the changes of the detected variable, and noise is generated thereby.